1. Field of the Invention
The present invention relates to a device that diagnoses the deterioration of oxidation catalyst used for an exhaust gas purifier of an internal combustion engine.
2. Description of the Related Art
Recently, emissions regulation of diesel engines has been tightened. Accordingly, removing Particulate Matter (hereinafter also referred to as “PM”) from exhaust air is desired by trapping PM included in the exhaust air from a diesel engine and burning the trapped PM. In this regard, an engine includes a particulate filter (a Diesel Particulate Filter, hereinafter also referred to as “DPF”) to trap the PM at its exhaust passage. The DPF is made of a material such as ceramic and stainless steel. To trap the PM, the DPF includes, for example, a honeycomb structure. If the PM is excessively trapped, the DPF becomes clogged. Therefore, as one method, the DPF includes an oxidation catalyst (Diesel Oxidation Catalyst, hereinafter also referred to as “DOC”) at an upstream location. The DOC oxidizes NO in the exhaust air to NO2 using an oxidation catalyst. Use of the NO2 thus generated allows the DOC to also oxidize and burn the PM trapped by the DPF. Thus, the PM is removed to continuously regenerate the DPF.
The performance of the DOC to generate NO2 deteriorates over time due to usage. Accordingly, the regeneration performance of the DPF is also deteriorated. Therefore, a device that diagnoses the deterioration degree of the DOC has been developed (see JP-A-2012-36860).
First, this diagnosis device estimates a NOX value in the exhaust air immediately after being discharged from the engine based on engine revolution and an engine load. Also, the NOX value at the downstream side of the DOC is directly detected by a NOX sensor installed at the downstream of the DOC. Next, a NO2 ratio is calculated using these two NOX values. The concentration of NO2 that has passed through the DPF is obtained from the NO2 ratio. The deterioration degree of the DOC is determined from this NO2 concentration.